The present invention relates to the field of an improved electroslag strip overlay system. More particularly, the invention relates to an improved system for creating high quality strip overlays in confined spaces such as pipe, fittings, or any hollow sections.
Weld overlays are formed on the surface of pipe, pipe fittings, pressure vessels, or pressure vessel components to resist corrosion, abrasion, or chemical embrittlement. Submerged Arc Strip Overlaying ("SAWSO") techniques have been used for many years to perform weld overlays. In SAWSO systems, a permanent electric arc is established between a consumable strip and the base material. The electric arc continuously "bites" into the base material to generate a mixed molten pool of base material and consumable strip. Flux is introduced into the molten pool in front of and behind the consumable strip to facilitate the formation of molten slag which protects and yields a proper deposition of the consumable strip onto the base material.
Although SAWSO techniques were created for overlaying large surfaces such as the interior surfaces of pressure vessels, various techniques have been developed to overlay relatively small diameter components such as pipe fittings. Prior work by the present inventor was disclosed in U.S. Pat. No. 5,134,268 to Capitanescu (1992) wherein weld strips were sequentially bonded to a pipe interior, and in U.S. Pat. No. 5,205,469 to Capitanescu (1993), wherein a submerged arc welding tip followed the radius of a base material.
Electroslag strip overlay processes ("ESSO") provide an alternative overlay process to SAWSO techniques. ESSO techniques melts a consumable strip through resistance heating formed by electric current transmitted through a shallow molten pool of electrically conductive slag. ESSO techniques do not use electric arcs as do SAWSO techniques, and flux is only fed in front of the consumable strip. ESSO techniques provide the advantages of high productivity, smooth surfaces, and few required repairs, and yield overlay deposits with half of the dilution rates experienced by SAWSO overlays.
Early attempts to use ESSO techniques for high temperature, high pressure, and hydrogen atmosphere applications resulted in failures where the overlay material disbonded from the base material. Hydrogen disbonding occurs with high pressure, high temperature operations such as in hydrocracking, hydrosulphurization and catalytic reforming where pressures can exceed 12-15 Mpa and temperatures can exceed 500 degrees C. Because the flux is introduced ahead of the consumable strip in ESSO techniques, the trailing jaw is exposed to molten slag. This generates light too bright for an operator to observe without eye protection, contact between the trailing jaw and the hot molten pool can melt or distort the trailing jaw if the jaw cooling system is disrupted, and significant spattering of molten slag occurs which leads to deposits on nozzles, flux guides, and adjacent overlays. Additionally, ESSO systems are not available for overlaying pipe and fittings having interior diameters less than thirty inches.
Efforts to overcome these problems have generated additional problems. If flux is added behind the consumable strip, shallow marks on the overlay deposit are generated by entrapped gases which are unable to escape through the molten slag. As the heavy burden of flux rests on top of the molten slag, gases are formed and are unable to escape through the slag layer.
There is, therefore, a need for an improved overlay system capable of eliminating the problems described above. The system should be efficient, should produce a high quality overlay, and should be usable within the narrow confines of small diameter pipe and pipe fittings interiors.